1. Field of the Invention
The present invention relates to hard disk drives (HDDs). More particularly, the present invention relates to a technique for reducing Non-Repeatable Run-Out (NRRO) of an actuator arm assembly of an HDD caused by high-frequency actuator and arm (actuator/arm) modes.
2. Description of the Related Art
FIG. 1 shows an exemplary hard disk drive (HDD) 100 having a dual-stage servo system for positioning a slider assembly 101 over a selected track on a magnetic disk 102 for writing data to and/or reading data from the surface of disk 102. The dual-stage servo system of HDD 100 includes a primary actuator 104, such as a rotary voice-coil motor (VCM), for coarse positioning an actuator arm 105 and a read/write head suspension 106, and a secondary actuator (not shown in FIG. 1), such as a microactuator or micropositioner, for fine positioning slider assembly 101 over a selected track. A microactuator (or a micropositioner), as used herein, is a small actuator that is typically placed between a suspension and a slider and moves the slider relative to the suspension, but can be placed on the suspension or other locations within a dual-stage servo system. Slider assembly 101 includes a read/write head (not shown) having a read element and a write element that respectively read data from and write data to disk 102. While HDD 100 is shown as having only a single magnetic disk 102, HDDs typically have a plurality of stacked, commonly rotated rigid magnetic disks and a corresponding number of actuator arms, read/write head suspensions, secondary actuators and slider assemblies.
FIG. 2 depicts a cross-section of an exemplary suspension and rotary microactuator arrangement 200 that can be part of a dual-stage servo system. Suspension and microactuator arrangement 200 includes a suspension 201, a microactuator 205 and a slider 209. Suspension 201 includes a load beam 202, a dimple 203 and a flexure 204. Microactuator 205 includes a substrate 206, a microactuator structure 207 and at least one flexure element 208. Substrate 206 is the stationary structure of microactuator 205. Microactuator structure 207 is the movable structure of microactuator 205. Slider 209 includes a read element 210 and a write element 211 that is offset from read element 210.
FIG. 3 is a schematic block diagram showing an exemplary actuator arm assembly 301 that can be used for the actuator arm assembly shown in FIG. 1. Actuator arm assembly 301 includes a primary actuator 302 (corresponding to VCM 104), an actuator arm portion 303 (corresponding to actuator arm 105), a read/write head suspension portion 304, (corresponding to suspension 106) and a slider assembly 305 (corresponding to slider assembly 101).
Actuator arm assembly 301 is controlled by an exemplary conventional control system 306 that includes a control circuit 307 that generates a signal 308 that is output to a primary amplifier 309 that, in turn, drives primary actuator 302. When primary actuator 302 is a rotary-type VCM, actuator arm assembly 301 rotates (as indicated by arrows 310), about a pivot 311 under the force generated by primary actuator 302. Control circuit 307 also generates a signal 312 that is output to a secondary amplifier 313 that, in turn, drives a secondary actuator (not shown in FIG. 3). A position signal 314 representing the position of slider assembly with respect to a disk is input to control circuit 307.
The position of a read/write head in relation to data on a disk is affected by the effects of VCM 302, external disturbances 315, resonant modes of actuator arm assembly 301, and motion of the disk. FIG. 4A is a graph of Non-Repeatable Run-Out (NRRO) as a function of frequency based on representative data that is typical for the currently available generation of actuator arm assemblies. FIG. 4B is a graph of cumulative NRRO as a function of frequency corresponding to the graph of FIG. 4A. The abscissa for both FIGS. 4A and 4B is the frequency, and the ordinate of both FIGS. 4A and 4B is NRRO. It should be understood that all of the graphs depicted herein are simulations that are based on data that is representative for the currently available generation of actuator arm assemblies for the enterprise class of hard disk drives. It should also be understood that graphs depicted herein could be based on data that is representative for other currently available classes of hard disk drives, such as desktop hard disk drives, mobile hard disk drives and consumer electronics hard disk drives. In FIG. 4A, NRRO caused by operational vibration (an external disturbance 315) is indicated at 401. NRRO caused by disk flutter (another external disturbance 315) is indicated at 402. NRRO caused by high-frequency actuator and arm (actuator/arm) mode effects of actuator arm assembly 301 is indicated at 403. The large amplitude motion at the high-frequency resonances of actuator arm assembly 301 may result in the inability of the read/write head to read or write data at the appropriate location on a disk.
Many of the resonant modes of actuator arm assembly 301 are greater than the bandwidth of the control loop for VCM 302 because the control bandwidth of VCM 302 is generally limited to be below the first main resonance of VCM 302. For example, FIGS. 5A and 5B respectively show a magnitude and phase response as a function of frequency for primary amplifier 309 and VCM 302. The first main resonance, or primary mode, of VCM 302, commonly referred to as the butterfly mode, is indicated at 501 in FIG. 5A. Higher-frequency actuator and arm (actuator/arm) modes corresponding to high-frequency actuator/arm mode effects 403 in FIG. 4A are indicated at 502 in FIG. 5A.
FIGS. 6A and 6B respectively show a magnitude and phase response as a function of frequency for the primary control portion of conventional control circuit 307, that is, the portion of control circuit 307 that controls VCM 302. FIGS. 7A and 7B respectively show an open-loop magnitude and phase response as a function of frequency for the primary control portion of control circuit 307, primary amplifier 309 and VCM 302. The butterfly mode can be observed at 701 and the higher-frequency actuator/arm modes can be observed at 702. FIG. 8 shows a closed-loop magnitude response of the VCM error rejection as a function of frequency for the primary control portion of control circuit 307, primary amplifier 309 and VCM 302. At higher frequencies corresponding to the frequencies of the actuator/arm modes, portions of the VCM open-loop frequency response corresponding to the higher-frequency actuator/arm modes are greater than 0 dB, as indicated by 702 in FIG. 7A. This generally results in portions of the VCM error rejection corresponding to the higher-frequency actuator/arm modes that are less than 0 dB, as indicated by 801 in FIG. 8. Magnitudes of the error rejection frequency response that are less than 0 dB indicate desirable disturbance rejection. Higher-frequency actuator/arm modes that have VCM open-loop frequency response magnitudes that are greater than 0 dB, however, are difficult or impossible to stabilize and often lack robustness to manufacturing tolerances, parameter variations, and other factors. Thus, a conventional primary control loop for a VCM does not adequately compensate for higher-frequency actuator/arm modes.
One conventional approach to reduce the NRRO that occurs at the higher-frequency actuator/arm modes has been to use notch filters in the primary control loop to prevent primary actuator 302 from exciting the resonant modes of actuator arm assembly 301. FIGS. 9A and 9B respectively show a magnitude and phase response as a function of frequency for the primary control portion of control circuit 307 when control circuit 307 includes notch filters. The effect on the frequency response by the notch filters are shown at 901. FIGS. 10A and 10B respectively show an open-loop magnitude and phase response as a function of frequency for the primary control portion of control circuit 307, primary amplifier 309 and VCM 302 when control circuit 307 includes notch filters. The attenuating effect of the notch filters on the higher-frequency actuator/arm modes is shown at 1001. FIG. 11 shows a closed-loop magnitude and phase of the VCM error rejection as a function of frequency for the primary control loop portion of control circuit 307, primary amplifier 309 and VCM 302 when control circuit 307 includes notch filters. The magnitudes of the VCM open-loop frequency response at the higher-frequency actuator/arm modes are well below 0 dB, as indicated by 1001 in FIG. 10, so there are no stability issues associated with the higher-frequency actuator/arm modes and the higher-frequency actuator/arm modes should be excited only weakly by the motion of the actuator arm assembly 1201. The magnitudes of the VCM error rejection frequency response at the higher-frequency actuator/arm modes, however, is nearly flat at 0 dB, as indicated by 1101 in FIG. 11. This means that the higher-frequency actuator/arm modes are very susceptible to excitation by other types of disturbances, such as airflow.
Thus, the decreased open-loop gain caused by the notch filters decreases the disturbance rejection of control loop for primary actuator 302 at the frequencies of the higher-frequency actuator/arm modes, thereby making the resonant modes more susceptible to excitation caused by external disturbances 315. One technique to increase the disturbance rejection at a particular frequency is to introduce a peak filter. See, for example, U.S. Pat. Nos. 6,339,512 and 6,487,028, both to Sri-Jayantha et al. Introducing peak filters in to the primary control loop at the higher-frequency actuator/arm modes, however, would cancel the stabilizing effects of the notch filters and suffer the same instability and robustness issues as the case without the notch filters shown in FIGS. 7A and 7B.
Another technique to improve disturbance rejection is to increase the open loop bandwidth. Increasing the open loop bandwidth of the primary control loop also has limited effectiveness in reducing the adverse effects of the higher-frequency actuator/arm modes. The Bode Integral Theorem mathematically proves that all feedback loops have a region of disturbance attenuation and a region of disturbance amplification. See, for example, H. W. Bode, Network Analysis and Feedback Amplifier Design, Princeton, N.J.: Van Nostrand, 1945. Moreover, the ratio of the attenuation to the amplification regions is fixed. Consequently, regardless of how a higher bandwidth is achieved for the primary control loop, the disturbance amplification region will still exist and will generally be pushed to a higher frequency. Because the bandwidth of the primary control loop is generally limited to be below the butterfly mode, the negative affects of the higher frequency actuator/arm modes will potentially be exacerbated.
Yet another approach for reducing NRRO is based on design of shrouding and mechanics for reducing actuator/arm mode excitation caused by airflow, but often provides limited effectiveness and may have negative implications on other aspects of the HDD.
Consequently, what is needed is a technique for reducing the adverse effects of high-frequency actuator/arm modes, while maintaining stability and robustness, thereby reducing total off-track motion of a read/write head and enabling a higher track density for an HDD.